Because an insulation material of an electric apparatus is usually deteriorated with time, it is desirable to monitor the deterioration and take measures. A leakage of electricity due to a dielectric breakdown should be prevented by an earth leakage breaker. It would be preferable if it is possible to monitor deterioration in insulation, foresee a dielectric breakdown, and prevent the dielectric breakdown. The earth leakage breaker can detect a leak current from an apparatus to the ground. However, because a change in insulation resistance due to the deterioration of the insulation material is little, it is difficult to measure the change. For example, in a machine tool used in a production site of a factory, a plurality of motors, such as a spindle motor, a servo motor and the like are used. Insulation materials between housings and coils of the motors are deteriorated with time. In such a machine tool, usually, when a large leak current flows because of deterioration in insulation of motors, an earth leakage breaker connected to a system power supply operates and the entire apparatus stops. In this case, because the apparatus suddenly stops, the influence on the production is large, it is difficult to specify a place of cause of a leakage of electricity, and it takes time to restore the apparatus. Therefore, there is a need for a mechanism for accurately detecting deterioration in insulation resistance to the ground or the housing of the apparatus or the load, for example, the motor or a mechanism capable of periodically detecting insulation resistance, foreseeing the deterioration, and performing preventive maintenance of the apparatus.
Patent Literature 1 discloses a technology for feeding an electric current to a load with a voltage stored in a smoothing capacitor but, rather than detecting the electric current, monitoring a change in the voltage of the smoothing capacitor and calculating insulation resistance from a time constant of the change. In the technology disclosed in Patent Literature 1, a measurement target is not a feeble current but is the voltage change of the capacitor in which noise less easily occurs. Therefore, it is considered possible to perform highly accurate measurement robust against noise.
Patent Literature 2 has an object of “obtaining a motor driving apparatus that can inexpensively foresee insulation deterioration of a motor” and discloses a motor driving apparatus in which “a closed circuit of a housing, a motor coil, resistance R1, resistance R2, a relay contact K1, diodes D4, D5, and D6, an alternating-current power supply 1, and a ground G1 of a motor 10, which is connected to a ground G2, is formed by turning on the relay contact K1 when a motor driving amplifier 8 does not operate. Consequently, a voltage to the ground of the alternating-current power supply 1 is applied to the closed circuit. When the insulation resistance of the motor 10 is high, an electric current flowing to the closed circuit is small and a potential difference due to the resistance R1 is small. When insulation of the motor is deteriorated, a leak current increases and a potential difference of the resistance R1 exceeds a reference voltage determined by a Zener diode. Then, an output signal is output from a comparator 32, a signal is output from a photo-coupler 35, and a decrease of an insulation resistance is displayed on a display of a control device 11. It is possible to easily and inexpensively foresee insulation deterioration of the motor and prevent a sudden operation stop by a leakage of electricity or the like”. In the technique disclosed in Patent Literature 1, the insulation resistance of a motor of a machine tool is measured from the side of a driving apparatus.